block: use memcpy_from_bvec in __blk_queue_bounce

Rewrite the actual bounce buffering loop in __blk_queue_bounce to that
the memcpy_to_bvec helper can be used to perform the data copies.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20210727055646.118787-14-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/bounce.c

@@ -239,24 +239,19 @@ void __blk_queue_bounce(struct request_queue *q, struct bio **bio_orig)
 	 * because the 'bio' is single-page bvec.
 	 */
 	for (i = 0, to = bio->bi_io_vec; i < bio->bi_vcnt; to++, i++) {
-		struct page *page = to->bv_page;
+		struct page *bounce_page;
 
-		if (!PageHighMem(page))
+		if (!PageHighMem(to->bv_page))
 			continue;
 
-		to->bv_page = mempool_alloc(&page_pool, GFP_NOIO);
-		inc_zone_page_state(to->bv_page, NR_BOUNCE);
+		bounce_page = mempool_alloc(&page_pool, GFP_NOIO);
+		inc_zone_page_state(bounce_page, NR_BOUNCE);
 
 		if (rw == WRITE) {
-			char *vto, *vfrom;
-
-			flush_dcache_page(page);
-
-			vto = page_address(to->bv_page) + to->bv_offset;
-			vfrom = kmap_atomic(page) + to->bv_offset;
-			memcpy(vto, vfrom, to->bv_len);
-			kunmap_atomic(vfrom);
+			flush_dcache_page(to->bv_page);
+			memcpy_from_bvec(page_address(bounce_page), to);
 		}
+		to->bv_page = bounce_page;
 	}
 
 	trace_block_bio_bounce(*bio_orig);